1. Field of the Invention
The invention relates to a thermal marking device for marking surfaces of webs of mineral wool which contain a binder.
2. Background of the Related Art
Such a device is known for example from DE-OS 37 13 108. In this publication a device is described which applies to the surface of a web of mineral wool markings which extend transversely to the longitudinal direction or run of the web. For this purpose, the device comprises a roller having on its periphery heated bars, the form and disposition of which correspond to the markings which are to be applied. The roller is rotated at such a speed that the heated bars which are constructed as marking bars roll their plane surface, which lies tangentially to the axis of the roller, over the surface of the web of mineral wool. The bar-like marking sections consist of metal and are heated electrically to such a temperature that upon contact with the surface of the web of mineral wool a flat zone of decomposition is created which corresponds to the contours of the marking bars and in which the binder in the mineral wool web decomposes and, by becoming discolored, forms an optically visible marking image in the form of a line.
However, if the length of the marking bars extends over a relatively large area of the mineral wool web, problems arise with regard to heat expansion. For this reason, the displacement of the bar-like marking elements in a longitudinal direction is limited. This avoids the heat expansion from the marking elements causing the marking bars to be deformed since the displacement path is restricted, but by reason of bulging or the like and therefore irregular action of heat, inaccuracies can occur in the marking pattern applied to the web of mineral wool.
The marking image which is created by the device according to DE-OS 37 13 108 represents an interrupted line or what may be referred to as a broken line. To form markings other than short axially parallel lines, a heating bar with its marking bar can be bent to suit the desired marking. However, in addition to excessive penetration of the outer edges of the marking bars into the surface of the mineral wool web, and thus localized overheating and fuzzy outlines to the marking, this can result in excessive structural complication and the need to provide minimum gaps, e.g. gaps between the letters in the case of inscriptions. Designs which are more complex and which contain intersecting lines, on the other hand, especially by reason of the concentrated supply of heat to the intersections and the resultant excessive temperature, can result in increased action of heat on the binder, resulting in at least partially illegible markings, and the entire marking device being subject to constant mechanical stress due to differing heat expansion, so that a high rate of wear and tear on the parts affected must be anticipated.